[1] I highly doubt it's Ulquiorra. There is simply no possible way for him to return.
I don't know what version you read, but on mangareader I looked up the chapter and Ulquiorra said, and I quote, "If you do not cut me down now, then this fight will forever remain unsettled."
Emphasis on the "forever", implying this was their last battle, and also the last opportunity for Ichigo to settle it - meaning, Ulquiorra is dead and will not return.
I'm seeing other people commenting it being Yoruichi or Isshin. This does not make sense. The way the person is standing next to Urahara and the facial expressions Urahara makes implies this is a non-friendly, threatening person. Why would either Yoruichi or Isshin do that?
This mystery person is either an entirely new character, possibly Nel, or probably Grimmjow.
[2] Perhaps, but it was a brutal way of killing someone and you can see the brute force utilized. I think you seem to misunderstand something about Grimmjow's character - he enjoys killing people who rival him in strength or form a threat to him. Typically, that would be the moment he'd laugh. If the mystery person is Grimmjow, it wouldn't be out of character at all. Obviously Opie wouldn't be a threat to him and rather than enjoying the kill and laughing about it, Grimmjow would see Opie more as an obstacle between him and whatever it is that he wants, so he decides to clean him up. The sword at Urahara's head would be a show of dominance and power. All in all, it's something that I could very easily see him doing.
Ulquiorra would probably not use as much force and show of strength to simply kill one opponent. I doubt he'd even bother to move all the way over to Kisuke either, he would simply stand near Opie and look at Urahara from a distance. Ulquiorra does not feel the need to establish his dominance, it's not like him to either.
About it not being yoruichi or isshin becuz he looks scared part..note:he is not looking Up at the person face an they just appeard in front of him in a flash he is surprised more than scared.